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Rotational Motion question

2020 · 8 Jan · Shift 2 · Q42
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  5. /2020 · 8 Jan · Shift 2 · Q42

Rotational Motion question

2020 · 8 Jan · Shift 2 · Q42

JEE MainPhysicsRotational MotionMCQ+4 / −1
A uniform sphere of mass 500 g rolls without slipping on a plane horizontal surface with its centre moving at a speed of 5.00 cm/s. Its kinetic energy is :
  1. A
    8.75 × 10–3 J
  2. B
    1.13 × 10–3 J
  3. C
    8.75 × 10–4 J
  4. D
    6.25 × 10–4 J
View written solutionFree

Correct answer: C

  1. Given data
  • Mass of sphere: m=500 g=0.5 kgm = 500\text{ g} = 0.5\text{ kg}m=500 g=0.5 kg
  • Speed of centre: v=5.00 cm/s=0.05 m/sv = 5.00\text{ cm/s} = 0.05\text{ m/s}v=5.00 cm/s=0.05 m/s
  • The sphere is uniform and rolling without slipping.
  1. Kinetic energy of a rolling body

For rolling without slipping, K=Ktrans+KrotK = K_{\text{trans}} + K_{\text{rot}}K=Ktrans​+Krot​

So, K=12mv2+12Iω2K = \frac{1}{2}mv^2 + \frac{1}{2}I\omega^2K=21​mv2+21​Iω2

For a uniform solid sphere, moment of inertia about its centre is I=25mR2I = \frac{2}{5}mR^2I=52​mR2

And for rolling without slipping, ω=vR\omega = \frac{v}{R}ω=Rv​

  1. Substitute into the rotational part

Krot=12(25mR2)(vR)2K_{\text{rot}} = \frac{1}{2}\left(\frac{2}{5}mR^2\right)\left(\frac{v}{R}\right)^2Krot​=21​(52​mR2)(Rv​)2

Krot=12⋅25mv2=15mv2K_{\text{rot}} = \frac{1}{2}\cdot \frac{2}{5}m v^2 = \frac{1}{5}mv^2Krot​=21​⋅52​mv2=51​mv2

Thus total kinetic energy is K=12mv2+15mv2=710mv2K = \frac{1}{2}mv^2 + \frac{1}{5}mv^2 = \frac{7}{10}mv^2K=21​mv2+51​mv2=107​mv2

  1. Put the values

First, v2=(0.05)2=0.0025v^2 = (0.05)^2 = 0.0025v2=(0.05)2=0.0025

Then, K=710(0.5)(0.0025)K = \frac{7}{10}(0.5)(0.0025)K=107​(0.5)(0.0025)

K=0.7×0.5×0.0025K = 0.7 \times 0.5 \times 0.0025K=0.7×0.5×0.0025

K=0.35×0.0025=0.000875 JK = 0.35 \times 0.0025 = 0.000875\text{ J}K=0.35×0.0025=0.000875 J

K=8.75×10−4 JK = 8.75 \times 10^{-4}\text{ J}K=8.75×10−4 J

  1. Match with options

8.75×10−4 J8.75 \times 10^{-4}\text{ J}8.75×10−4 J corresponds to Option C.

  1. Comparison with stored answer

Stored correct answer: C

My derived answer: C

They agree.

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